Downhole drilling requires a mechanism for rotating a drillstring. Traditionally, this rotation was created by a rotary table. However, rotary tables have largely been replaced by topdrives, which have the advantage of permitting the drilling to be done with three joint strands rather than a one joint strand.
Hydraulically-powered motors are typically used (although electric motors may instead be used) to rotate the topdrive and thereby impart rotation to the drillstring. Typically, in the case of a hydraulic motor, a pump pressurizes and circulates working fluid through a manifold, which controls fluid flow into the motor of the topdrive.
In two-speed prior art control systems for hydraulic motors using a four-port motor having two fluid intakes and two fluid outputs, the hydraulic fluid may be simultaneously be pumped into the motor's two intakes and out through the motor's two outputs. This arrangement is used when low speed, high torque is required from the motor, for example when the drill is moving through high density substances. Alternatively, when the drill is moving through low density substances, the motor is run at high speed, low torque. However, during high speed, low torque operation, the manifold is typically arranged so that one of the manifold intakes and one of the manifold outputs is “dead ended”. This disadvantageously results in a loss of energy due to vicious flow, creation of heat, and friction loss.
A real need thus exists for a hydraulic motor control that is capable of providing working fluid to a hydraulic motor that operates at low speed, high torque or at high speed, low torque, and that can switch between the two operating conditions without power loss.